Where is a mockingbird referenced in To Kill a Mockingbird?

The Word mockingbird is only in the book To Kill a Mockingbird once or twice.

Only twice. The title comes from theses quotes: "Mockingbirds don't do one thing but make music for us to enjoy... but sing their hearts out for us. That's why it's a sin To Kill a Mockingbird."
- Harper Lee, To Kill a Mockingbird

'Remember it's a sin to kill a mockingbird.' That was the only time I ever heard Atticus say it was a sin to do something, and I asked Miss Maudie about it."
- Harper Lee, To Kill a Mockingbird